why do teabags swell up and get full of air when I’m pouring hot water onto them?
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tl;dr – Pouring the water over the teabag makes it air tight, but liquid can still flow in and out. The plant matter has tiny amounts of air inside of it, and it will come out as it rehydrates. The hot water has gas dissolved in it, usually, and it can also get into but not out of the bag.
The way you pour/dip the bag into the water. Yes. This can cause it. If you just pour the water in fast, then it can ‘trap’ a bubble of air inside the now moist bag. Same as your shorts when you go in the pool. However, if you dip slowly and carefully and let the water soak into the bag… It still has a bubble of air in it after a few minutes. So it has to be inside the actual plant matter.
The plant matter is dried, and it contains pockets of air where water used to be before it was dried, and it expands as it is warmed up by the hot water. So even if you are perfect with the dipping technique, you can’t get all of the air out from the plant matter. The bag itself will become wet and seal in that air, long before it has time to expand and escape the plant material.
And the last one is that the hot water is usually boiled pretty recently, and as such might have a lot of dissolved gas inside of it. Unlike carbonation, where it is forced into the liquid, this is far less total volume of gas. Just enough to stick to plant material and/or the bag. The outside of the bag can shed off bubbles, but the inside of the bag is going to have tiny amounts of dissolved gas slip through, and then find a place on the plant matter inside to become a bubble. Tiny, tiny amounts of gas. But it is enough.
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